A depth camera system obtains data regarding the location of a human or other object in a physical space. This information may be referred to as “depth information.” The depth information may be input to an application in a computing system for a wide variety of applications. Many applications are possible, such as for military, entertainment, sports and medical purposes. For instance, depth information regarding a human can be mapped to a three-dimensional (3-D) human skeletal model and used to create an animated character or avatar.
To determine depth information the depth camera may project light onto an object in the camera's field of view. The light reflects off the object and back to the camera, where it is processed to determine the depth information. One technique for determining distance to the object is based on the round trip time-of-flight of the light. Some techniques modulate the light and determine distance based on the phase difference between the transmitted and received light.
However, aliasing may occur with some time-of-flight techniques due to the periodicity of the modulated light. Aliasing occurs because it is possible for the light returning from objects at two different distances to have the same phase shift relative to the transmitted light. Stated another way, the phase shift of light reflected from some objects may be greater than 360 degrees, with phase shifts greater than 360 being indistinguishable from phase shifts less than 360 degrees. The term “depth window” may be used to refer to the depth that can be detected without encountering the aliasing effect. For example, the depth window could be 2.5 meters for a given camera.
One de-aliasing technique is to modulate the light at several different frequencies. For each modulated frequency, the measurement indicates that the object could be at one of several possible distances. One of the distances is correct, and the others are due to aliasing. However, by analyzing the set of possible distances for all modulated frequencies the correct distance may be determined. However, this technique requires that the light be modulated at several different frequencies. Therefore, additional time is required to collect the data. Also, the circuitry may need to be more complex to be able to modulate the light at different frequencies. Also, even with a few different modulated frequencies it is not guaranteed that the aliasing effect can be removed.
Therefore, further refinements are needed which allow a more accurate determination of the depth of objects within a field of view of a depth camera. Improved techniques for de-aliasing when performing depth detection are desired. The techniques should be compatible with existing depth detection cameras.